Position information is used in a wide range of fields, such as a navigation system and a map information service. Most famous systems as a means for acquiring position information include a Global Positioning System (GPS).
In the GPS, radio waves transmitted from a plurality of GPS satellites are received by one receiver, and a position of a reception device is estimated from a time difference in arrival of the radio waves. Each of the GPS satellites is equipped with an atomic clock in order to accurately obtain an arrival time difference of radio waves, and further, radio waves can be transmitted with accurate timing by correcting the atomic clock regularly. However, generally, such an accurate clock is extremely expensive and is difficult to be equipped on a terminal side in terms of cost.
Patent literatures 1 to 6 (PTL1 to PTL6) disclose techniques of estimating a position of a wireless device that constitutes a wireless network system.
Techniques disclosed in PTL1 and PTL2 are for estimating a position of a mobile station by using a reception delay time of radio waves in a mobile communication system. In the technique in PTL1, a position is estimated by using an arrival time difference of radio waves from a plurality of base stations. In addition, in the technique in PTL2, a position of a mobile station is calculated by measuring a delay time of radio waves from a mobile station, the position of which is desired to obtain, to a plurality of base station antennas disposed in a periphery of a zone.
PTL3 discloses a technique of estimating a position of a mobile station by measuring received radio wave intensity from a plurality of base stations. A method of estimating a position by using radio wave intensity has an advantage of being low in cost in comparison with other methods. However, since the radio wave intensity is largely affected by am ambient environment, the method also has a problem of being extremely low in position estimation accuracy. PTL3 improves, by using a neural network, the problem of being low in position estimation accuracy in the method of estimating a position by use of radio wave intensity.
PTL4 discloses a method of estimating each position of a plurality of sensor nodes from a result of measuring a distance between the sensor nodes. The technique disclosed in PTL4 uses a method of enhancing accuracy of overall position information by eliminating a low-reliability location estimation value by obtaining, from a distance measurement result, an angle between position-known anchor nodes and comparing the angle with a predetermined threshold value.
PTL5 is a technique related to a wireless device capable of autonomously estimating a position of a wireless device that constitutes a wireless ad-hoc network. According to PTL5, a wireless device determines an own position by autonomously correcting a temporary own position in sequence, in such a way that a computed distance computed based on the temporary own position and a temporary position of a neighboring wireless device approaches a measured distance between the own and the neighboring wireless device.
PTL6 discloses a distance measurement method based on a direct spread spectrum scheme.